From the Super Mario Wiki
Flashes are normal elemental ghosts and a sub-species of the Purple Puncher found in Luigi's Mansion. Ice protects the hearts of these white ghouls. Luigi must melt these ice hearts by capturing a Fire Elemental Ghost and spewing fire at the ghost while avoiding its uppercut. Then, the Flash will become vulnerable to the Poltergust 3000. The Flash only appeared in the Hidden Room, Kitchen and Roof, although two of them are encountered in the Butler's Room during the blackout along with two Gold Ghosts. The Flashes first appeared in the Hidden Room in all except three chests.